Antenna elements of the kind that includes the present invention are called micro strip antennas or patch antennas and constitute an all round class of antenna elements. Hence, the basic building block of these antennas is a radiation disc in the shape of a flat, metallic plate. The plate was a shape that, in principle, is arbitrary, that is a patch, separated from a ground plane by means of air or a dielectric. By correctly choosing the dimensions of the plate it is made resonant for a chosen electromagnetic radiation frequency. Usually, the frequency, is in the microwave range. The plate can be used for absorbing or radiating energy at this frequency. It is often desirable that the antenna absorbs or radiates radiation solely having a particular polarization. It is required that the antenna has a good purity of polarization (a low cross polarization, a low axial ratio). It is often required that the antenna can obtain two orthogonal polarizations corresponding to two antenna ports, that is, two linear or opposed elliptical, in particular circular polarizations, having a good purity of polarization. It is clear that deviations from a geometrical symmetry can cause degradation of the polarization purity. The supply itself to the radiation disc can cause such an asymmetry. The problem related to polarization purity then becomes pronounced in the case when the antenna element is made a broad band, type since the effect from the supply in this case becomes marked.
A number of different solutions of the problem relating to polarization purity in antenna elements of the above type has been suggested.
The problem of geometrical asymmetry caused by the supply of the radiation disc can be solved by means a balanced supply, but this will add supply points. This is a complication that can not always be accepted. Balanced supply results in a doubling of the number of supply points for the antenna element, an increased complexity and the problem with potential build-up.
Cutting-outs and tongues at the edges of the antenna elements are often used to obtain a circular polarization using only one supply point. In the case of two polarizations they can, in principle, be used in order to improve the polarization purity. In practice it seems to be hard to design such radiation discs having cutting-outs and tongues since they also modify the electric dimensions of the antenna.
If the antenna element is to generate a circular, single or double that is, right and left rotating polarization and a quadrature hybrid is used, unbalance in the hybrid provides a method of improving the polarization purity. Unless circular polarisation is required and the hybrid already is a part of the antenna, the use of unbalanced hybrids seems to be of little value.
Sequential rotation of and supply to elements can be used as a method of improving the polarization purity in an array of antenna elements. It is a supplement to other methods and only works in the case when the antenna elements are used together with other elements in an array where cross-polarization of different antenna elements can be made to cancel each other at angles close to the broad side direction.